ABSTRACT Systemic lupus erythematosus (SLE) is a prototype of systemic autoimmune disorder that affects mostly young females. It causes significant morbidity and mortality. Effective therapies without significant side effects for induction of remission and prevention of relapses are wanting. Our novel hypothesis that links the HLA-D region to the pathogenesis of SLE is that autoantibodies (auto-Ab) and auto-reactive T cells are generated by the activation of T cells cross-reactive with autoantigens (auto-Ag) and enviromental (such as bacterial) Ag in the HLA-DR restricted manner. Over a period of time accumulation of auto-reactive T cells leads to the production of complex auto-Ab, resulting in clinical disease in suitable hosts. Remission occurs with reduction of auto- reactive T cells and of auto-Ab specificities. Relapses occur with repeated stimulation by molecular mimics in the host. The initial clinical presentation and the relapses depend on the nature of the stimulation by the molecular mimics. We have extensively mapped the T cell epitopes in a lupus-related Ag, SmD. This auto-Ag has cross reactive intra- and inter-molecular T cell epitopes. The presence of intra-molecular cross-reactive T cell epitopes is the reason why they are targeted in SLE. The presence of multiple cross-reactive T cell epitopes among polypeptides of snRNP provides us the understanding of the mechanism of B cell epitope spreading in SLE. Multiple T cell antigenic regions have been identified in SmD. Each region appears to induce unique patterns of auto-Ab specificity. Multiple T cell receptors (TCR) are utilized in responses to immunization with SmD. Several microbial mimics from commansal flora were identified with the capability of inducing diverse auto-Ab in a similar manner as the parental peptide. There are cross reactive B cell epitopes between the mimics and the parental SmD peptides. Cross reactivity between tetenus toxoid (TT) T cell epitopes and those of SmD have been documented. These results and other preliminary data provide the basis for the current proposal for us to understand better the role of environmental Ag in the induction of SLE-related auto-Ab. Three specific aims are proposed: Specific Aim 1: To translate our observations in our DR3 transgenic model to normal individuals and SLE patients and to relate SLE-related Ab induction to microbiota and tetanus toxoid; Specific Aim 2: To demonstrate that antibiotic treatments that deplete or modulate the gut microbiota will modulate autoantibody production and/or disease course with prolonged survival in (NZM2328xNOD)F1, NZM2328 and NZM2328.DR3; and Specific Aim 3: To derive germ-free mice from NZM2328 and NZM2328.DR3 to show that lupus nephritis and lupus related auto-Ab will not develop in a germ-free or gnotobiotic environment, supporting the thesis that microbiota play a crucial role in the development of SLE-related auto-Abs. On the basic level, the expected results will provide insight to the origin of auto-Ab in SLE and will provide evidence that both T and B cells play important roles in SLE. They will also support targeting microbiota as a novel approach to treating and preventing the development of SLE.